Generally, it is very important to insure gas permeability and liquid permeability in a blast furnace for producing pig iron during its operation into which coke (generic name for iron ore, sintered ore, lime stone, and the like) are loaded therein. When gas permeability in a blast furnace becomes lower, increase of pressure loss or non-uniformed gas flow may occur with defective descent of the burden (frequent occurrence of hanging and slip). which in turn not only makes the operation unstable but also lowers a reaction efficiency in the entire furnace as well as productivity of the blast furnace. Furthermore, when the liquid permeability becomes lower, slag overburden is generated at the tuyere level, which causes not only non-uniformity in gas distribution in the furnace, but also tap hole deviation and a rise in the pressure of in the furnace, thereby causing a non-uniform tap output rate from each tap hole. This phenomenon also causes defective decent of the burden and damages the operational stability of the furnace. In relation to the gas permeability and liquid permeability in a blast furnace, it has been recognized that the operational factors, such as gas permeability and liquid permeability are especially important in the core section. The core section comprises a lower section of the tuyere level and a core coke layer existing under a zone where the ores are softened and melted (Refer to FIG. 1). The function of the core section 7 is to control gas flow distribution in a furnace, and as a result, its construction effects the stability and descent of the burden. When the furnace utilizes pulverized coal injection, the core section 7 serves as a path for unburned materials to pass from the tuyere, up to the softening and melting zone.
In order to provide the proper heat source, reducing capacity, gas distribution (gas permeability), liquid permeability and the dropping of molten metal and slag, a relatively high quality coke for the blast furnace has been used. Apart from the possible problem of future exhaustion of feed stock coal used for producing such high quality blast furnace coke, there is the problem that blast furnace coke typically has a high porosity, or a low compression strength or a low strength after reaction (CSR) nature. Even in a case where the coke for a blast furnace has a relatively higher quality than that of commercial coke, the coke can become powdered due to various types of physical or chemical phenomena generated in the furnace. For this reason, it is difficult to completely stabilize the operations of a blast furnace and to improve the gas and liquid permeability by only using a high quality blast furnace coke.
An attempt in overcoming the problems described above, is found in Japanese Patent Laid-Open Publication No. 63206/1978. The disclosure discusses a method of operating a blast furnace for which coke is used, characterized in that 3 to 25% of the total charged coal materials by weight is replaced with high strength block made of fine carbonaceous materials, and where coke fine materials are mixed with the coke for use in the blast furnace.
With that method however, as the fines and high strength coke was charged into the furnace in place of the ordinary coke, the gas permeability was temporally improved, but the high strength block intruded into some areas other than the core section of the furnace. That condition lowered the furnace reaction efficiency of the entire furnace. Furthermore, the high strength blocks descended to the raceway section in front of the tuyere, which in turn, caused incomplete combustion of the coke, and in addition, oxygen to climb to the upper side of the furnace, which in turn caused the FeO-rich slag to drop to the raceway section, causing that section to become unstable. All of these conditions made it difficult to stabilize operations of the blast furnace.
Another disclosure discussing the technology for preserving the gas permeability and the liquid permeability in stable condition as well as for enhancing furnace operational stability, is found in the "Method for controlling a solid reducing bed in a furnace core during operations of a blast furnace" disclosed in Japanese Patent Laid-Open Publication No. 65207/1989. In this publication, the method disclosed is one to control the gas permeability and liquid permeability through the coke layer, which is continuously updated in association with proceeding of the blast furnace operation, and use of a solid reducing agent. The solid reducing agent is charged into the core section of the ore layer and the solid reducing agent is charged into the core section as a solid reducing agent layer, and simultaneously the core section of the layer is specified as inside of the core section area in the furnace, where the relation as indicated by the expression of r.sub.t .gtoreq.0.03 R.sub.t is satisfied, where the solid reducing agent to be charged into the core section, is charged such that the agent charged into the specific areas occupies 0.2% or more by weight of the total weight of the solid reducing agent charged into the entire core section. Herein, the variable R.sub.t indicates a radius of the furnace top section, and the variable r.sub.t indicates a set radius from the furnace core, in the furnace top section.
In that methodology however, high-quality coke with high hot/cold compression strength and adjusted granularity is always charged into and used in a central portion of the furnace, so that although it can be expected that the gas permeability and liquid permeability will be improved to some extent as compared those in the conventional technology, the effect is practically the same as that in a case where only typical blast furnace coke is used, and for this reason substantial improvement of the gas permeability and liquid permeability can not be expected. It is suggested in the publication that silicon carbide bricks or graphite bricks or the like, each with a low reactivity may used in place of high-quality coke. Regardless of what type of bricks are charged into the furnace, it is predicted that the same problems as those relating to the technology disclosed in Japanese Patent Laid-Open Publication No. 63206/1978 may occur, and for this reason, there are still some questions left as to whether the operation can fully be stabilized or not.
On the other hand, injection of pulverized coal into a blast furnace is known to be an effective alternative to the use of a high quality r educing agent, but injection increases the production of fine materials in the gas circulating inside the furnace and unburnt materials deposited in the core section, causing the gas distributing function to become disturbed, which in turn worsens the gas permeability as well as the liquid permeability. Accordingly, with pulverized coal injection, the stable operation is still uncertain, and it is said that the coal injection rate is limited to tip to 200 kg/ton-pig so long as the current type of blast furnace coke is used for commercial operation. For the reasons described above, it is an object of the present invention to substantially improve the gas permeability and liquid permeability inside the core section of the furnace in order to stabilize furnace operation for the injecting of pulverized coal. Furthermore, it is another object to use a large quantity of low grade solid reducing agent instead of the high quality coke.